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How to Plant Onions: Ultimate Guide

If you’re planning on adding onions to your next season’s garden strategy, there are some facts you’ll need to know about planting and harvesting them.

Some varieties will grow better in various regions and climates. While these are general guidelines, reading about specific types for your area is best for a good outcome. 

Below is a basic guide on how to plant onions. 

Growth Cycle

How long do onions take to grow? Onions take approximately 90 days to reach the harvesting stage after planting, depending on growing conditions.

When deciding to plant from seed, it’s best to start them indoors in a tray or container. Onion sets, which are further along in the process, maybe another option.

Planting the Seeds 

Start the seeds in a container with rich compost or potting soil. You can also soak them first for better germination. Also, follow the packaging details as instructions may vary by climate and variety.

Seeds should be retained in a warm and moist area until they sprout and are mature enough for transplantation. Transplant when the seedlings reach four inches tall.

See also: How to Plant Garlic: A Basic Growing Guide

Planting From Sets 

Onion sets are miniature bulbs that, once put into the soil, will develop into full-size bulbs after about three and a half months.

These are a bit more robust and grow much faster than seeds. Plant approximately 3 inches apart in rows 10 inches apart.

Also, for best results, soak the sets in compost tea for 15 minutes before planting.

When picking up bulbs at a nursery, you should check that they are dry (typically bought in mesh bags) and firm.

Checking for leaf and bulb health before planting is essential for the eventual output of the mature plant. 


Spacing will be dependent on planting from seeds or sets.

If going for seeds, start them six weeks before ground planting, in containers, and then space approximately four to five inches apart when transplanting.

When doing sets, plant them about three inches apart. Also, add straw mulch between sets, and in between the rows, allow for 12 to 18 inches of spacing. 

Planting Depth 

Sink seeds about half an inch below the soil level.

First, it is crucial to amend the soil with compost and other organic material to support the onion’s growth.

If planting from sets, place them approximately one inch down into the soil where they can take root.

Remember, the type of onion will also determine how deep you’ll put it in the ground. It is crucial to know your climate and variety to fully understand its growing needs. 

Water Requirements

Provide about an inch of water per week. Also, it is best to utilize mulch around the plants as it ensures proper saturation without making the earth too soggy around these plants.

Well-drained soil is necessary for these plants to best mature.

Rot is a real consideration, which is why ensuring the best mulch or moisture barriers without drowning the plant is critical. 

Growing From Scraps 

It is possible to grow onions from onions.

  • Cut off the bottom and put toothpicks or other materials that can hold the scrap in place over a cup or bowl.
  • Fill the dish, jar, or container with water and suspend the scraps above, and water the source.
  • Soon you’ll start to see tiny sprouts coming out the top and root-like tentacles from the bottom.
  • Place the sprouted scrap into the ground or a pot, covering until just the sprouts are above the soil line.
  • Water, drain, and cultivate as you would any other onion that is grown from seed or set.

Growing in Containers 

You can grow onions in shallow containers of at least eight inches, either from sets or seeds.

  • Fill the container about halfway with soil, and keep it watered but well-drained once planted.
  • Add rich compost until the bulbs appear.
  • The bulb should remain at least an inch under the soil, and if exposed, should be recovered.
  • Leaves can be cut down to help the plant to focus more fully on the development of the bulbs.

Companion Plants 

Anything in the cabbage family makes a great companion plant for onions.

They help to deter aphids, Japanese beetles, and rabbits, so anything that these pests enjoy can be married with the onion.

Additionally, lettuce and carrots, for instance, have very different root systems that won’t compete for the same resources under the soil.

Tomatoes, lettuce, and strawberries, to name just a few other examples, are great companion plants.

Strawberries, for instance, benefit from being close to onions, which help deter diseases that impact that plant.

Avoid all varieties of peas and beans as there is a chemical incompatibility with these vegetable families.

Finally, chamomile is a plant that can actually help to improve the overall flavor, so that’s a great companion plant to add to your garden. 


How do you know when onions are ready? Firstly, the top green leaves that start to wilt is an indication that they are near harvesting. The bulbs will continue into maturity as the leaves start to lose their color.

This occurrence is due to the plant now putting all reserves toward the aging of the bulb itself. If any have flowers on the stalks, they should be harvested immediately.   

When harvesting, dig or pull on the tops to safely remove the whole plant from the soil. Next, shake the dirt off from around the bulbs and ready them to be cleaned and stored.

Let the bulbs sit in the garden for 1-2 days to allow natural drying.

Additionally, you may wish to loosen the dirt and bring it back from the bulb to allow drying before harvesting.

Curing by allowing them to dry will be evident as their outer layers tighten around the bulb and dry. 

Pest and Diseases  

  • Onion fly is one of the most prolific pests that affect the entire allium family. These lay eggs and hatch maggots that feed upon the seedlings. This fly resembles the house fly. After 3 days of egg-laying, they mature and burrow their way into the onion.
  • The lesser bulb fly looks similar but is smaller. Once infected by them, the plants must be dug up and destroyed.
  • Thrips are another insect that lays eggs on the leaves and then sucks the plant sap.

There may also be many diseases due to uncontrolled watering, resulting in lesions and rot signature on the plant.

Diseases include neck rot, purple blotch, botrytis leaf blight, damping off, downy mildew, green mold, and more. 


Store onions in dry and cool locations. If too moist or warm, resprouting can occur.

Mesh bags, bushel baskets, or blat cardboard boxes with holes provide the best storage methods to keep them dry and cool for the short term.

Keep in mind that onions will only last about seven to ten days before thoroughly drying out or resprouting even in the best of conditions.

This is true in most mild varieties but may be extended weeks or even months in proper conditions for pungent types.

Again, consider your climate and your storage options for the best crop outcome.


Planting onions in your garden is a rewarding venture. Whether you will start with seeds or use small bulbs of sets, various factors will help determine the length of growth.

Additionally, you will need cool storage and dry conditions until you can, cut, or otherwise enjoy your harvest.

Sasha Brown

Blogger and lover of all things natural.

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